It is known that in order to decommission land used for military purposes and return it to a safe, habitable environment, all unexploded ordnance (UXO) remaining in the ground must be removed safely and efficiently. This needs to be accomplished with painstaking and hazardous hand work.
Current technology provides both passive and active systems for the detection of subterranean unexploded ordnance. The passive systems measure variations in the earth's magnetic field and are the safest to human personnel as they do not result in the triggering and detonation or "functioning" of unexploded ordnance. They do not, however, provide an accurate, sensitive scan as they are subject to environmental interference, such as anomalous magnetic soils, and are unable to detect non-ferrous metals.
Active "time domain" metal detectors are more accurate, detect non-ferrous metals, and probe deeper than do the passive systems. An electromagnetic transmitter produces a pulsed primary magnetic field in the earth, which induces eddy currents in nearby metallic UXO. The eddy current decay produces a secondary magnetic field measured by the receiver coil. The measurement is taken at a relatively long time after the start of the decay to allow the current induced in the ground to fully dissipate. In this way, only the current produced by the secondary magnetic field is detected by the receiver coil. The responses from the receiver coil are recorded and displayed by an integrated data logger.
Time domain metal detectors are most efficient when operated by human personnel as their location on the ground must be precisely controlled in order to relocate UXO once the data collected by the data logger is evaluated.
The electrical current and resulting magnetic current produced by the active metal detecting devices also induces an electrical current in nearby metallic UXO. This current may be sufficient to trigger fuses of unexploded ordnance that employ electronic fuses, with catastrophic results for the person conducting the search.
Other inventors have attempted to clear UXO, and more specifically mines, by triggering electronic fuses with electromagnetic signals. Because the nature of a signal that will trigger a fuse is unknown, a variety of signals are tried. For example, U.S. Pat. No. 5,361,675 to Specktor describes a land-based invention wherein a magnetic mine detonation apparatus is mounted on a vehicle and utilizes varying waveform configurations for the purposes of detonating all magnetic mines within the scan area.
U.S. Pat. No. 4,220,108 to Burt describes a multi-sweep method in an underwater environment. While minesweeping ships are constructed with a very low magnetic signature, modern magnetic detection systems have advanced to the point where even these small magnetic signatures can be detected. Burt teaches use of a permanent magnet which produces a large magnetic signal, greater than the magnetic signature of a following minesweeping ship. This permanent magnet is towed ahead of the following minesweeper with the objective of actuating substantially all magnetically triggered mines in its path. As is virtually impossible to duplicate the exact magnetic signature characteristics of a minesweeper, the field strength of the first magnetic signal is varied by allowing the magnet to move zigzag back and forth transverse to the course and clear a channel. The minesweeper follows relatively safely in the de-mined channel. Additional minesweeper or additional passes may be used to actuate mines in an ever-widening channel.
To function UXO, two aspects must be satisfied: one, a signal must be produced which is capable of triggering functioning of the UXO; and second, the signal must be generated in the location of the UXO. Note that each UXO is unique in terms of its design, historical trauma and environmental effects, and as such, signal characteristics which are capable of triggering functioning of the UXO are unknown.
The prior art minesweepers address a similar problem with magnetically-activated mines. As part of the solution, they vary the waveform as they traverse a mined area. One of the risks with a moving system and a variable waveform is that there is no assurance that waveform being applied at any instant is the one which would trigger the fuse in mine being scanned.
The prior art minesweepers further seek to actuate or function as many of the mines as possible and, for safety, space the mine-triggering device some distance from the personnelcarrying vehicle.
The prior art operate on a macro scale for clearing a path through a field for passage of additional minesweeping or other military vehicles and are not concerned with accurate, detailed detection of UXO. The problem remains that for decommissioning of land subsequently intended for human use, there is a demonstrated need to be able to safely remove all UXO, utilizing the precision which is only possible using human-operated detecting equipment or extremely expensive, remote vehicle-navigation systems such as inertial guidance and sub-cm differential global positioning systems.